A “compression ratio” of an internal combustion engine is defined as the ratio of the volume in a cylinder above a piston when the piston is at bottom-dead-center (BDC) to the volume in the cylinder above the piston when the piston is at top-dead-center (TDC). The higher the compression ratio, the more the air and fuel molecules are mixed and compressed, resulting in increased engine efficiency. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the engine.
In conventional internal combustion engines however, the compression ratio is fixed for all cylinder units and cannot be changed individually to accommodate temperature and other cylinder state differences. This notwithstanding, some variable compression ratio (VCR) internal combustion engines have been developed to vary the clearance volume of a cylinder in order to achieve improved fuel economy and increased engine power performance in some engines mechanically. Such VCR engines are designed to have a higher compression ratio during low load conditions, and a lower compression ratio during high load conditions. Known techniques include using “sub-chambers” and “sub-pistons” to vary the volume of a cylinder, U.S. Pat. Nos. 4,246,873 and 4,286,552, varying the actual dimensions of all or a portion of a piston attached to a fixed length connecting rod, U.S. Pat. No. 5,865,092, and varying the actual length of a connecting rod, U.S. Pat. No. 5,724,863.
Other techniques for constructing variable compression into internal combustion engines include the use of eccentric rings or bushings either at the lower “large” end of a connecting rod or the upper “small” end of the connecting rod for varying the effective length of the connecting rod or height of a reciprocating piston. U.S. Pat. Nos. 5,417,185, 5,562,068 and 5,960,750 and Japanese Publication JP-03092552 disclose devices that include eccentric rings. These eccentric ring devices, however, are undesirable in that each eccentric ring must be rotated 180 degrees before one of the desired operating modes or positions is engaged. As a result, locking of the eccentric ring in a proper position may not occur within an optimum period of time, thereby leaving the effective length of the device and consequently the compression ratio of an associated cylinder in an undesired intermediate state.
U.S. Pat. No. 6,668,768 describes a connecting rod assembly that may be transitioned between two or more compression modes without requiring rotation of an eccentric ring member about a crankpin or wrist pin. The connecting rod assembly of the invention is configured to vary a compression ratio of an internal combustion engine having a crankshaft, a piston and special assemblies with a first portion connected to the crankshaft and having a cylindrical aperture with a second portion adapted to be connected to the piston and movable with respect to the first portion. In addition, the assembly includes a locking element movable between an unlocked position and a locked position for locking the second portion at a first position relative to the first portion. U.S. Pat. No. 6,675,087 describes a variable compression ratio apparatus with compression ratio operating modes based in part on driveline surge, surge tolerance, compression ratio operating modes. Others propose hydraulic methods to extend the connecting rods to provide compression ratio variability or multi-link type piston-crank mechanism enabling a compression ratio to be varied by changing attitude of the links.
The common innovative thread in all the above solutions is that they are all mechanical changes to the engine or engine components. The incremental costs are additional mechanical contrivances for reconfiguring rods, crankshafts and engine heads, hardware assemblies, mechanical process gyrations. The benefits are fixed designs with a limited range of discrete compression ratio operating modes. What is needed are more expansive and flexible ways to change the compression ratio in an internal combustion engine in real-time. What are needed are more precise methods of changing a compression ratio over a larger range of compression ratios and methods of controlling changing compression ratios through a continuous range that can be selected and implemented dynamically.
Engine Developments
The internal combustion engine has seen thousands of improvements and developments. Some of the latest improvements include fuel efficiency, pollution reduction, electronic ignition, fuel mixture heating or cooling, fuel injection, variable displacement, air-fuel mixing and digital controlling of hydraulically actuated intake/exhaust valves. Camless hydraulically driven intake and exhaust valves and electronically controlled hydraulic fuel injectors are among the very latest innovations to impact internal combustion engines.
A computer control system which provides commands to electronic assemblies can finely control and vary valve actuation, fuel injection and ignition. Electronic assemblies process commands and feedback signals from these devices to manage engine operation. Camless valve control allows engine control subsystems to vary timing, lift, and compression ratio in response to engine load, temperature, fuel/air mix, and other factors. The electronic valve-control system improves performance while reducing emissions.
There are several methods of camless valve control. Sturman, U.S. Pat. No. 6,360,728 Control Module for controlling hydraulically actuated intake/exhaust valves and fuel injection, claim fast-acting electro-hydraulic actuators which provide mechanical means for valve actuation under the control of an electronic assembly. Solenoid actuated two-way spool valves can also be actuated by digital pulses provided by an electronic assembly. Camless technology brings the internal combustion engine under even more electronic control potential and away from inflexible mechanical controls.
Spark Ignition (SI) Engine
Spark ignition (SI) engine operation involves ignition of a homogeneous or stratified mixture of air and readily vaporized high octane fuel, such as gasoline, using an electrical discharge (spark) from one or more ignition devices such as a sparkplug, located in the combustion chamber of the engine.
Ignition and combustion of the air/fuel mixture in SI engines is relatively slow, particularly at low loads, resulting in less than optimal thermal efficiency and fuel efficiency since only a portion of the fuel's energy is released at the point of maximum compression. Combustion of the air/fuel mixture begins at the spark plug (under normal operating conditions). Since the flame has a single flame front, a finite period of time, which is dependent on many factors, is required for the flame (generated by the spark at the sparkplug) to propagate across the combustion chamber. The air/fuel mixture furthest from the spark plug is ignited substantially later than the air/fuel mixture near the sparkplug. During flame propagation the pressure in the combustion chamber increases. The compressed air/fuel mixture furthest from the flame front is compressed to higher and higher values awaiting the flame. If the compression pressure and corresponding temperature of the air/fuel mixture awaiting the flame is sufficient, as well as the exposure time, the air/fuel mixture will auto-ignite before the flame reaches it. Auto-ignition of the air/fuel mixture results in very rapid rates of combustion generating high combustion pressures, rates of combustion pressure rise and combustion knock, which may cause engine damage depending on many factors. SI engines employ high-octane fuels to minimize auto-ignition of the air/fuel mixture.
Ever since first working four-stroke engines, a fundamental limitation the internal combustion reciprocating engine has been the volume swept out by the piston has had to remain fixed. Thus, engine designers have had to build engines tuned to work most efficiently during periods of high loading. Since a vehicle does not spend all its time climbing hills or racing away from a stop, much of the time the engine is operating below peak demand, and below peak efficiency. Sweden's Saab Automobile SA conceived a concept they call the SVC engine
Saab invented the “monohead,” a way to vary cylinder volume without resorting to variable height pistons or eccentric connecting rod bearings. This assembly combines cylinder head and cylinder walls in a single unit. The monohead pivots on a pin inside the crankcase. On the side opposite the pin, an actuator rotates a cam, driving connecting rods to cant the monohead by as much as 4 degrees. Tilting the monohead to any spot along 4 degrees of arc adjusts engine compression to an infinite number of ratios between 8:1 and 14:1. These limitations are set by the tilting angle and supercharger air delivery.
Compression ratio sets the level to which a piston compacts a mix of air and fuel before a spark ignites it. A typical car engine runs at a fixed compression ratio, somewhere around 9.3:1. If the ratio could be changed continuously, an engine would run more efficiently at light loads with a reduced incidence of ignition knock at heavy loads. Increasing the compression ratio to as much as 14:1 for light loads is ideal, he said. For heavy loads, dropping the compression ratio as far as 8:1 nearly stops engine knock, the damaging agent in engines, altogether.
In mechanical designs using variable compression with a fixed stroke length, a naturally aspirated engine could only surpass the efficiency of a conventional engine by 4 or 5 percent. Saab found that they needed boost pressure from a supercharger, to appreciable increase engine power with variable compression. However, this was shown for SI engine, not compression ignition or homogeneous charge compression ignition or other types of engines.